(CNN) -- As horrified travelers watched, a Greyhound Canada bus passenger repeatedly stabbed and then decapitated a young man who was sitting and sleeping beside him, a witness said Thursday.

"There was a bloodcurdling scream. I was just reading my book, and all of a sudden, I heard it," Garnet Caton, who was sitting in front of the two men, said of the Wednesday night incident west of Portage la Prairie in Manitoba.

"It was like something between a dog howling and a baby crying, I guess you could say," Caton said. "I don't think it will leave me for a while."

Passengers exited the bus, and a trucker who stopped provided wrenches and crowbars to several of them so they could keep the suspect on the bus until police came, witnesses told Canadian TV.

The suspect was seized with the help of negotiators, Royal Canadian Mounted Police Sgt. Steve Colwell said. Watch Colwell discuss the case »

He said no formal charges had been filed, and he declined to identify either the man in custody or the victim, who were among 34 passengers.

There was no immediate indication of what prompted the attack, Colwell said. He said he didn't know how many times the victim was stabbed. Witnesses described the weapon as a large butcher-type knife.

Caton told The Associated Press that the victim appeared to be about 19 years old and had gotten on the bus in Edmonton.

Colwell praised the "extraordinary" level-headedness and bravery of the bus driver and passengers.

"What you saw and what you experienced would shake the most seasoned police officer. And yet I'm told that each of you acted swiftly, calmly and bravely," Colwell said. "As a result, no one else was injured."

The police received a call reporting the attack at 8:30 p.m. By the time they arrived at the scene, everyone except the knife-wielder and his victim had left the bus, Colwell said. The incident ended about 1:30 a.m.

The bus was traveling along the Trans-Canada Highway from Edmonton, Alberta, to Winnipeg, Manitoba, and was about 45 minutes from its destination when the attack occurred, Greyhound spokeswoman Abby Wambaugh said in Dallas, Texas.

Caton said the victim was sleeping with his head leaning against the window when the attack happened. Caton said he shouted at the other passengers, many of whom also were sleeping, to leave. Watch Caton describe what he saw »

"Everybody got off the bus. Me and a trucker that stopped and the Greyhound driver ran up to the door to maybe see if the guy was still alive or we could help or something like that," Caton said.

"And when we all got up, we saw that the guy was cutting off the guy's head. ... When he saw us, he came back to the front of the bus, told the driver to shut the door. He pressed the button and the door shut, but it didn't shut in time, and the guy was able to get his knife out and take a swipe at us," Caton said.

Caton told the AP that the attacker didn't sit near the victim when he first got on the bus, about an hour before the attack.

"He sat in the front at first; everything was normal," Caton said. "We went to the next stop, and he got off and had a smoke with another young lady there. When he got on the bus again, he came to the back near where I was sitting. He put his bags in the overhead compartment. He didn't say a word to anybody. He seemed totally normal."

Half an hour later, the attack began, Caton told the AP. "There was no rage or anything. He was like a robot, stabbing the guy."

The incident occurred on the first of two Greyhound Canada buses that were traveling together, Wambaugh said. The bus was carrying 37 passengers. As many passengers as possible among those not directly involved in the incident were transferred to the second bus, she said.

Others were taken to a hotel in Brandon, where they were met by Greyhound managers and police, Wambaugh said.

Once they are released, Greyhound will take them by bus to Winnipeg, and "we will do whatever is required to help them, and that includes counseling," she added.